Investment Securities - Amortized Cost, Gross Unrealized Gains and Losses and Fair Value of Securities Held to Maturity (Detail) - USD ($) $ in Thousands	Jun. 30, 2015	Dec. 31, 2014
Schedule of Held-to-Maturity Securities [Line Items]
Held to Maturity, Amortized Cost	$ 1,720,575	$ 1,778,316
Gross Unrealized Gains	44,437	56,974
Gross Unrealized Losses	(25,717)	(19,314)
Held to maturity, fair value	1,739,295	1,815,976
U.S. Treasury securities
Schedule of Held-to-Maturity Securities [Line Items]
Held to Maturity, Amortized Cost	139,051	139,121
Gross Unrealized Gains	11,025	12,179
Gross Unrealized Losses	0	0
Held to maturity, fair value	150,076	151,300
U.S. government agency securities
Schedule of Held-to-Maturity Securities [Line Items]
Held to Maturity, Amortized Cost	13,523	14,081
Gross Unrealized Gains	279	304
Gross Unrealized Losses	0	0
Held to maturity, fair value	13,802	14,385
Obligations of states and state agencies
Schedule of Held-to-Maturity Securities [Line Items]
Held to Maturity, Amortized Cost	195,735	197,440
Gross Unrealized Gains	7,226	9,410
Gross Unrealized Losses	(1,114)	(412)
Held to maturity, fair value	201,847	206,438
Municipal bonds
Schedule of Held-to-Maturity Securities [Line Items]
Held to Maturity, Amortized Cost	302,393	302,578
Gross Unrealized Gains	9,142	10,955
Gross Unrealized Losses	(983)	(278)
Held to maturity, fair value	310,552	313,255
Obligations of states and political subdivisions
Schedule of Held-to-Maturity Securities [Line Items]
Held to Maturity, Amortized Cost	498,128	500,018
Gross Unrealized Gains	16,368	20,365
Gross Unrealized Losses	(2,097)	(690)
Held to maturity, fair value	512,399	519,693
Residential mortgage-backed securities
Schedule of Held-to-Maturity Securities [Line Items]
Held to Maturity, Amortized Cost	965,647	986,992
Gross Unrealized Gains	14,280	18,233
Gross Unrealized Losses	(10,379)	(6,244)
Held to maturity, fair value	969,548	998,981
Trust preferred securities
Schedule of Held-to-Maturity Securities [Line Items]
Held to Maturity, Amortized Cost	79,617	98,456
Gross Unrealized Gains	92	167
Gross Unrealized Losses	(13,241)	(12,380)
Held to maturity, fair value	66,468	86,243
Corporate and other debt securities
Schedule of Held-to-Maturity Securities [Line Items]
Held to Maturity, Amortized Cost	24,609	39,648
Gross Unrealized Gains	2,393	5,726
Gross Unrealized Losses	0	0
Held to maturity, fair value	$ 27,002	$ 45,374